During aerospace vehicle operations, knowing the amount of fuel the vehicle carries can be critical. For example, when a vehicle operates over an extended range or period of time, small variations in fuel consumption can determine whether the vehicle will be safely recovered or lost. Accordingly, various methods for determining the amount of fuel carried by an aerospace vehicle have been developed.
For example, some aerospace vehicles monitor engine fuel flow during the operation of an aerospace vehicle to determine the amount of fuel used over time. The amount of fuel used is subtracted from the total amount of fuel that was loaded on the vehicle prior to departure to provide an estimation of the amount of fuel remaining. A drawback of this method is that certain malfunctions cannot be detected. For example, a fuel leak would not be detected by the fuel flow indicators unless the leak occurs past the point were fuel flow was measured. Accordingly, an aerospace vehicle with an undetected fuel leak can run out of fuel even though the computed fuel remaining (based on the fuel flow indicators) indicates that the aerospace vehicle is still carrying a sufficient amount of fuel to continue to its recovery point.
Another method of determining the amount of fuel remaining in an aerospace vehicle during flight is to install pressure sensors in the fuel tank(s) of the vehicle. These pressure sensors measure the column pressure of the fuel in the corresponding tank. In many cases, a companion pressure sensor that measures atmospheric pressure is required to compensate for changes in column pressure due to changes in altitude. A drawback of these systems is that they are often large, bulky, and heavy. Additionally, these systems can require electrical components to be in contact with the fuel. In some cases, if these electrical components are exposed to fuel vapors, they can elevate the risk of explosion.